It is well known within the art to provide devices allowing heating steering wheels or other (curved) surfaces.
During construction of heating devices for a steering wheel the skilled person faces three major problems. If the heating device comprises wires, they can be visible, even if there are additional layers placed above the heating device, such as foam and natural or artificial leather. The wires can also be felt by the user. Reducing their thickness is not an option as, the wires are being subjected to strong stresses during the manufacturing process and while driving. These stresses often cause damage to the wires, which gives rise to an inhomogeneous heating, to the failure of the device or even worse to local overheating.
Some solutions propose the use of heating elements essentially comprising a rectangular shaped knit network of wires. When they are applied to a toroidal rim, some parts are stretched, while others are compressed. This gives rise to the inhomogeneity of the local density of the wires along with a significant heat inhomogeneity.
The electrical heating may, further, be achieved using a printed conductive layer or pattern. It can either be printed over the steering wheel rim itself, or can be first printed on a substrate and then applied to the rim. In the former case printing is difficult, and, therefore, expensive. In the latter case application of the substrate to the toroidal surface gives rise to substrate buckling or folding, which is undesirable, since it also can be sensed at least tactilely. Such an approach requires in any event specially shaped cuts to be applied to the substrate.
A further option is to apply a conductive paste directly onto the steering wheel rim making a certain pattern, thus avoiding the buckling/folding problem. This has, however, the drawback that application of the paste onto a complex toroidal surface is a work-consuming task increasing the steering wheel price.
All the above presented solutions strive to evenly distribute the heating sources along the steering wheel surface in order to generate the heat at the rim as homogeneously as possible. However, this, in itself, creates a problem, since the steering wheel not only has a complex toroidal surface, but there are more parameters which have to be taken into account, as determined by the present inventors and explained below.